The invention relates to a method and an apparatus for filtering data symbols for a decision based data processing system within a receiver.
FIG. 1 shows a conventional arrangement for data transmission. A transmitter transmits transmitted symbols SS to a receiver via a signal path or data transmission channel. The receiver receives the received symbols ES, which are processed by a data processing unit, for example. Signal interference means that the received symbols ES sometimes differ from the transmitted symbols SS. The transmitted symbols SS and the received symbols ES are data symbols comprising one or more data bits.
The receiver is a QAM receiver, for example. During data transmission via the real data transmission channel, the received signal ES normally has linear distortions and an additional noise component. The receiver takes the received signal ES and reconstructs the bit sequence from the data source. To this end, the analog received signal is first converted into a digital signal and is then sent to a mixing stage. A downstream reception filter suppresses any interference signals outside of the transmission frequency band. Suitable proportioning of a “matched filter” MF increases the reliability of detection as a result. The matched filter is a digital reception filter within the receiver, said digital reception filter being matched to a transmission filter within the transmitter such that the amplitude of the received signal is at a maximum at the sampling times. The output signal from the matched filter is supplied to a carrier phase detector TPD which is provided for carrier phase detection for a digital received signal. The carrier phase detector TPD sends a carrier phase detection error signal to a downstream digital loop filter. The digital loop filter and the downstream numerically controlled oscillator NCO deliver a digital control signal for a mixing stage. The frequency and phase estimation for the received signal ES can take place in one or two stages. In the case of a two-stage QAM receiver, said receiver contains a carrier frequency loop for detecting a carrier frequency for the received signal in a first carrier frequency reception range and a downstream carrier phase loop for detecting a carrier phase for the received signal in a second carrier frequency capture range.
The carrier phase detector TPD and the carrier frequency detector TFD are “decision based” systems (decision directed systems). The receiver contains a decision maker which compares the received symbols ES received with nominal symbols and associates the received symbol ES with that nominal symbol which is at the shortest interval from the received symbol ES.
FIG. 2 shows an example of the association between a received symbol ES and a nominal symbol ESnominal on the basis of the prior art. In the example shown, the nominal symbols S1, S2, S3, S4 have the values:    S1=+1, +1;    S2=−1, +1;    S3=−1, −1;    S4=+1, −1.
In the example shown, a received symbol ES=0.1; 0.1 is received by the receiver.
A decision maker calculates the intervals between the received symbol ES and the various nominal symbols S1-S4. In the example illustrated in FIG. 2, the intervals a1 are:    A12=1.62    A22=2.02;    A32=2.42;    A42=2.02.
The decision maker selects the minimum interval, which is A12=1.62 in the example. The probability of the transmitted symbol SS originally transmitted corresponding to the nominal symbol S1 is therefore greatest, and the received symbol ES=0.1; 0.1 is identified as nominal symbol S1=+1, +1.
FIG. 3 shows an example with 6 transmitted symbols, which are transmitted to the receiver by the transmitter via the data transmission channel and arrive at the receiver as a received symbol sequence ES. In the example illustrated in FIG. 3, the transmitter transmits a transmitted symbol SS1=−1, +1, for example, which is available to the receiver as received symbol ES2=+0.1, +0.1. As explained in connection with FIG. 2, the receiver makes a decision and wrongly identifies the received symbol ES2 as nominal symbol S1=+1; +1. This incorrect decision is supplied to the decision based system within the receiver, for example to a clock phase detector or to a carrier frequency detector. Decision based systems (decision directed systems) work well provided that the decisions supplied to them are correct. The decision based systems can also tolerate incorrect decisions provided that the incorrect decisions arise only in very rare cases and with very low probability. If the incorrect decisions occur more frequently or if the probability of incorrect decisions becomes greater, then the decision based systems, for example a carrier phase detector or a carrier frequency detector, fall out of step. In this case, a loop filter connected downstream of the carrier frequency estimator or clock phase estimator receives incorrect input values, which means that the mixing frequency which is sent by the numerically controlled oscillator NCO likewise has an ever greater level of error. The erroneous mixed signal causes the number of incorrect decisions to rise, which means that the control error increases further.